In a modern petrol-fuelled internal combustion engine, the petrol is injected into an intake duct in proximity to the cylinders (indirect injection) or is injected inside the cylinders (direct injection).
When the engine is hot, i.e. when the engine has reached a temperature close to the operating temperature, no significant differences in engine behaviour are observed when types of petrol with different volatility values are used. On the other hand, when the engine is cold and the outside temperature is chilly (for example below 10° C.), the non-volatile portions of the petrol remain liquid after injection and do not participate in combustion; in particular, the non-volatile portions of the petrol that remain liquid are deposited on the intake duct (in indirect injection engines) and on the walls of the cylinders where they are diluted with the lubrication oil or are discharged unburned via the exhaust valves. In order to compensate for the fact that only some of the injected petrol participates in combustion, it is necessary to increase the amount of petrol injected, i.e. it is necessary to “enrich” the fuel injection.
Enrichment of petrol injection is adjusted as a function of outside temperature (the lower the temperature, the more the injection has to be enriched), and as a function of petrol volatility (the lower the volatility of the petrol, the more the injection has to be enriched). The aim of the enrichment process is to provide the minimum enrichment that is sufficient in order to permit good starting of the engine, because any further enrichment merely increases the consumption of the engine and, above all, the output of pollutants.
The volatility of a particular type of petrol is a value that indicates the readiness of said petrol to pass from the liquid state to the gaseous state and is defined as the vapour pressure that is measured when the temperature of the petrol is 37.8° C. (state of equilibrium from liquid to vapour state). Thus, when expressed as dimensioned units, the volatility of a type of petrol is a pressure and is generally expressed as psi (“pounds per square inch”); 1 psi corresponds to 68.9 hPa.
The outside temperature is an item of data that is available in modern internal combustion engines either by means of direct measurement, or by means of measuring the temperature of the liquid coolant (when the engine is cold, the temperature of the liquid coolant is substantially equal to the outside temperature).
In contrast, the volatility of the petrol is an item of data that is only available in approximate terms, because it is excessively costly and complicated to install a sensor that is capable of directly measuring the volatility of petrol, and no sufficiently accurate and reliable method has yet been proposed for indirectly determining the volatility of petrol. In this connection, it is important to note that the volatility of petrol is variable both as a function of the refinery from which the petrol originates and as a function of the time of year; in the summer months, the types of petrol sold are much less volatile than those sold in the winter months. Cold starting of engines is indeed facilitated when the ambient temperature is high, but if the petrol is particularly volatile and the outside temperature is high when petrol is dispensed, a considerable amount of petrol vapour is formed, which is potentially harmful to the health of staff and to the environment. Normally commercially available types of petrol have a volatility of between 6 and 14 psi, i.e. between 413 and 965 hPa.
U.S. Pat. No. 6,079,396A1 discloses a method for compensating the fuel volatility during the cold start of an internal combustion engine. Automotive internal combustion engine fuel volatility is estimated during cold start operations by stabilizing air admission to the engine and analysing engine speed over a modelling period following an engine cold start after engine speed has stabilized and prior to closed-loop engine operation; if engine speed deviates significantly away from an expected engine speed for the current engine intake air and fuel, a fuel volatility deviation is diagnosed. The magnitude of the fuel volatility deviation away from a nominal fuel volatility is determined as a function of the magnitude of the engine speed deviation; a fuel volatility correction value is updated as a function of the engine speed deviation and is applied throughout an ignition cycle, including during the modelling period to compensate for the fuel volatility deviation
EP1178203A1 discloses an adaptive same-ignition-cycle passive method for the real-time detection and compensation of fuel volatility (or equivalently, the fuel driveability index) during cold start of a multi-cylinder engine. The method detects a signature of fuel volatility on engine speed immediately after engine starts to run; a local short-duration high-amplitude speed droop is associated with fuels of various volatility which can be detected within the first second after engine ignition while engine is in idle-neutral operation mode. The speed droop is uniquely correlated with the fuel driveability index value in the form of calibration tables at different temperatures; the actual fuel driveability index is thus detected and the optimum fuel enrichment/enleanment is quickly determined within few events after engine is flagged as running even before the transmission is engaged.